The present invention relates to temperature sensitive electrical switching devices and, more particularly, to a temperature sensitive device which is normally open but which closes an electrical circuit when the temperature of the device is raised to a predetermined temperature level.
Thermally actuatable switch constructions are presently known in which an electrical circuit is broken or closed in response to the switch being heated to a predetermined temperature level. In U.S. Pat. No. 3,875,546, issued Apr. 1, l975, to Merrill, and U.S. Pat. No. 3,519, 972, issued July 7, 1970, to Merrill, both assigned to the assignee of the present invention, temperature responsive electrical switches are disclosed in which a sliding electrical contact is held against a second electrical contact by a relatively stiff spring, which spring bears upon a normally solid, heat fusible non-conductive pellet. When the switch is raised to the required temperature and the pellet is melted, the stiff spring no longer opposes the force of a somewhat weaker spring which then moves the sliding contact away from the second contact, thus opening the electrical circuit.
A normally open electrical switching device is disclosed in U.S. Pat. No. 3,180,958, issued Apr. 27, l965 to Merrill and assigned to the assignee of the present invention. In this device, the heat fusible material holds the sliding contact away from a second contact until the fusion temperature is reached. While sliding contact, thermal switching devices of this type are extremely reliable in operation, there is always a possibility that the moving parts may bind. This is especially critical where the sliding contact is moved into contact with a stationary contact and must press firmly against the stationary contact in order to provide the desired low resistance electrical path through the switch.
A need exists, therefore, for a temperature sensitive switching device which is simple in construction but which provides for closure of an electrical circuit at a precise temperature level, regardless of the orientation of the device.